To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition of 75 (aVF), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price in previous auctions is 36.79 euros.
- The last auction with a similar condition (75) had a hammer price that is not specified, but the starting price was 20 euros.
- In previous auctions, hammer prices have varied, but the range has been between 24 and 40 euros for similar conditions.

3. Current Starting Price: The starting price is 20 euros, which may attract more bidders.

4. Price Trends: The average starting price in previous auctions is 23.96 euros, suggesting that bidders are willing to pay a little more than the starting price.

Based on these factors, an estimated maximum purchase price could be in the range of 35 to 45 euros, considering that the condition is good and that the hammer price history suggests that bidders have been willing to pay more in the past.

Therefore, a reasonable maximum purchase price could be 40 euros.